The activation of Factor X is a key step in coagulation and is likely to be a rate determining step in fibrin clot formation. Factor VIII, Factor IX and Factor X will be purified from human plasma. Continuous spectrophotometric rate assays will be used to monitor the rate of activation. These assays will enable the determination of the function of various factors involved in the reaction. The steady state kinetic parameters of the activation will be determined and the kinetic mechanism of the reaction will be deduced from these data. In this way we will determine the physiological regulatory mechanism of the Factor IX catalyzed activation of Factor X.